Power on/off a 3v toy with Arduino....

I’m trying to control (turn on and off) a 3-volt LED board (powered by two AAA alkaline batteries) using a 5 volt Arduino Pro Mini.

The controller board for the LEDs has a small unlabelled microcontroller on it, and is pretty low power (just 4 dim white LEDs). I can’t tell if it has any voltage regulation. A pic is attached (the pads at bottom center are + and -).

There’s a switch connecting the LED board to two AAA batteries, that interrupts positive.

I’m wondering what the best and/or easiest way to control that is? (And by “control” I mean simply turn it on and off).

  • Could I possibly use a resistor to power the LED board directly from the Arduino’s 5 volt pins?

  • Or would it require using a transistor? I have some TIP120 transistors and was considering using it as a project to learn to use those. If so, are these good instructions for this project?

  • Or if I used the 3.3 volt version of the Arduino Pro Mini could I likely power the board directly?

Thanks for any help.

The first question you must answer is how much current the board draws. Connect fresh batteries and insert your multimeter into the circuit to measure the current drawn.

If less than 30mA, then no transistor will be needed. If between 30mA and 600mA then a small transistor such as bc337. A tip120 is probably overkill.

To drop the Arduino's 5V supply to 3V: the transistor will drop around 0.7V. A couple of extra diodes like 1n4001 will get you down to 3V.

If less than 30mA, then no transistor will be needed. If between 30mA and 600mA then a small transistor such as bc337. A tip120 is probably overkill.

Looks like the current is only 11mA. In that case, can I bypass the toy's batteries (two AAA in series) entirely and power it from the arduino's 5 volt directly? And if so, is any resistor required?

I connected it briefly directly to the Arduino's 5 volt power line and it drew a lot more power (~243mA) so I'm guessing a resistor is required?

I connected it briefly directly to the Arduino's 5 volt power line

That was a pretty bad idea. All chips have maximum voltage ratings, and one that is designed to run on 2 AA batteries is unlikely to do anything but die, if connected to 5V.

You can damage (and may have damaged) the Arduino as well.

A much better idea, when replacing batteries with electronics, is to use an appropriate voltage regulator.

That was a pretty bad idea. All chips have maximum voltage ratings, and one that is designed to run on 2 AA batteries is extremely unlikely to do anything but die, if connected to 5V.

You can damage (and may have damaged) the Arduino as well.

Hope not! I wouldn't have done it if I didn't have backups of both, and was literally for half a second. Both are still working well though, for the moment at least.

Thanks for the tip on the voltage regulator. This one looks good to me, let me know if anyone thinks otherwise.

This one looks good to me

Looks like cheap junk to me, and it is an inefficient linear regulator. But, you could be the first to review it, and let other Amazon customers know what fraction of them actually work!

With Pololu, you get what you pay for, as well as product support.

Aha thanks for that.

Thanks for the tip on Pololu, I'll start using them.

Does this look like a more efficient step down regulator?

Edited to add: hmm, has a warning on the product page to use the D24V5Fx family. Which would be this one. Thoughts?

And on a sidenote, the Amazon reviews for what looks like the same regulator I posted before are hilarious. Capacitors falling off, output voltage suddenly jumping up to the input voltage, etc.

As the Nano's voltage is already regulated, you just need to drop around 2V to power your circuit. A resistor is not suitable, because the voltage it would drop would vary with the current drawn by the module. I was going to suggest 3 diodes in series, e.g. 1n4001 or 1n4004 etc. Each diode will drop around 0.7V and that won't vary much if the current varies.

hmm, has a warning on the product page to use the D24V5Fx family

That is not a warning, just an informational notice that more efficient versions are available. That matters for battery operated devices.

But a PaulRB mentioned, for this purpose you can probably get away with using 3 diodes in series to drop about 2 V.